The present invention relates to a method for the preparation of textured soybean draff or, more particularly, to a method for the preparation of textured soybean draff as a palatable high-protein foodstuff material.
Soybean is a very important raw foodstuff in Japan and other Oriental countries which can be processed into three major traditional foods including tofu, i.e., soybean curd, miso, i.e., fermented salty paste, and soy sauce. Tofu is a curd-like coagulation of the proteinous extract from soybeans in water separated by squeezing or pressing from the residue or soybean draff, which is called "okara" using the Japanese word. Okara contains large amounts of useful and nutrient proteins which are not extracted or which are less soluble in water. The amount of okara, i.e., soybean draff produced as a byproduct in the manufacture of tofu is almost equal by weight to that of the starting raw soybeans since the soybean draff contains water in a large percentage. Moreover, soybean draff is also produced as a byproduct in soybean processes other than the manufacture of tofu. The estimated annual yield of such soybean draff in Japan was as large as about 700,000 tons in 1983.
The soybean draff or okara is highly nutrient. It is typically constituted of 22% of proteins, 19% of fats, 38% of carbohydrates or saccharides, 15% of fibrous matter and 6% of ash on the dry basis (see, for example, Foodstuff Development, Vol. 19, No. 8, pages 11-18 (1984)). The protein utilization (NPU) of okara is 62% and even better than the corresponding value 45% of tofu per se. A problem in utilizing okara or soybean draff is the difficulty in handling due to the very high water content of 80% by weight or more. Dehydration of okara to decrease the water content is technologically difficult and economically almost impossible. Okara as a food has the disadvantage of extremely high susceptibility to putrefaction as a consequence of its high nutrition content and high water conent.
In recent years the trend has been that this nutritive food has less and less chance of use in the human diet. It is utilized almost exclusively as a feed for domestic animals and poultry. A very large part of the production of okara occurs in locations near big cities due to the location of the manufacturer of the principal food products which produce okara as the byproduct. The amount of the okara utilizable as a feed is limited because stock raising is usually not a principal industry in the vicinity of big cities. Therefore, a large part of the okara or soybean draff can find no other way of disposal than to be discarded as a waste material involving a serious problem of possible environmental pollution in the near future.
The aforedescribed situation relative to the disposal of this nutritional material (okara) is particularly unfortunate for those countries deficient in foodstuff resources or, in particular, in protein resources.
In the prior art, a textured protein of vegetable origin is prepared by the treatment of defatted soybeans or other material at an elevated temperature and under an applied pressure in a single-screw extruder machine. This process is suggestive of the mechanism that the protein molecules in the defatted soybeans having almost completely lost functional properties are further unfolded under the conditions of high temperature and high pressure produced inside the extruder so that the reactive groups of the protein molecules initially enveloped within the molecules have been exposed and reacted and a textured product is formed. By analogy to defatted soybeans, an attempt has been made for the texturization of okara by the above mentioned method using a single-screw extruder machine but the results obtained were unsatisfactory presumably due to the high water content of the okara.